Support assembly including a stress limiter for non-metallic cable, and method of fitting the stress limiter
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a support assembly for non-metallic cable comprising a metallic support fitting and a metallic stress limiter for relieving electrical stress between the cable and the support fitting, and is particularly useful with optical aerial cables which are positioned adjacent high voltage power lines. 
     2. Description of Related Art 
     As disclosed for example in a paper entitled &#34;Experience with Optical Fibre Aerial Cables on High Tension Power Lines&#34; by C. Jurdens, H. G. Haag and R. Buchwald, given at the Cigre International Conference on Large High Voltage Electric Systems, 1988 Session, non-metallic self-supporting overhead optical cables are supported adjacent towers at respective fixing points along the cable by means of preformed metallic fittings comprising helical wires which form a sheath around the cable and grip it tightly. Stress limiters in the form of toroidal control electrodes prevent corona discharge at the ends of the helical wires. These electrodes are commonly formed of wire wound into a toroidal cage whose centre ring connects mechanically and electrically with the ends of the cable support fitting furthest from the tower. 
     This known type of stress limiter is expensive to make and, more importantly, difficult to fit in situ. Further, it has been recognized that it is a combination of electrical and mechanical stresses at the interface of the cable and the support which causes degradation and ultimately failure of the cable in this critical area, where the cable is close to a power cable and where the optical cable support is grounded by the tower, and especially in weather conditions such that the optical cable is partly wet and partly dry. 
     SUMMARY OF THE INVENTION 
     The purpose of the invention is therefore to provide a support assembly incorporating a form of simple stress limiter which relieves both mechanical and electrical stress and is readily fitted in situ. 
     The invention provides a support assembly for non-metallic cable incorporating a preformed metallic stress limiter and a metallic support fitting in the form of helical wires which surround a region of the cable and grip it tightly, the stress limiter comprising a generally mushroom-shaped hollow metal body having a tubular stem portion located between one end of the support fitting and the cable, and at one end of the stem portion a hollow part-toroidal head portion consisting of the material of the stress limiter turned back over the adjacent part of the stem portion, and having an outer profile which is smoothly curving. The stem portion in use is inserted between the support fitting and the cable to distribute the mechanical force at the cable interface, and the head portion prevents corona discharge and effectively reduces the electrical tension. 
     The stress limiter may be formed as a single piece, but preferably in two halves divided along a diametric plane. Where the stress limiter is in two halves, there is the advantage in being able to fit it without first having to thread it over the cable. Where the halves are identical, this makes manufacture more economical and facilitates assembly. The stress limiter is preferably of sheet metal, conveniently a stainless metal such as stainless steel, the head portion being approximately in the form of a hollow toroid. 
     In such a case the wall of the head portion is preferably provided with at least one drain hole, and in use the stress limiter will then be fitted to a cable with the drain hole lowermost. In some cases the wall of the head portion may have a plurality of holes spaced at different positions around it so that at least one of the holes lies at or adjacent the bottom of the stress limiter and acts as a drain hole when the stress limiter is fitted to a cable. 
     The stem portion preferably has an unsmooth outer surface so that it is readily gripped in use by the cable support fitting. For example, the stem portion may be textured, splined or otherwise configured externally to provide said readily grippable surface. 
     There is preferably an annular gap between the stem portion and the head portion on the side designed to face the support fitting in use, to allow the end of the support fitting to be accommodated within the head portion, thereby minimising the electrical stress. 
     The part of the head portion closest the stem and defining the annular gap preferably has a sharp edge inclined away from the stem end to provide a barb engageable with the support fitting when the end thereof is accommodated within the head portion. 
     From a different aspect, the invention provides a method of fitting a stress limiter, in accordance with the first aspect of the invention, to a support fitting on an overhead cable comprising: temporarily loosening the support fitting; inserting the stem portion of the stress limiter into the annular gap thus formed between the end of the support fitting and the cable; and re-tightening the support fitting around the stress limiter. Where the stress limiter is formed with a gap between the head and stem portions, the stem portion is preferably pushed fully into the annular gap between the support fitting and the cable so that the end of the support fitting is accommodated within the head portion. 
     Where the support fitting comprises helically-wound wires, it may be re-tightened by rotation of the stress limiter, which grips the wires. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One way in which the invention is used will now be described, by way of example, with reference to the accompanying drawings, in which: 
     FIGS. 1 and 2 are perspective views from different ends of part of an aerial cable supported by a support assembly which includes a support fitting and a stress limiter, embodying the invention; 
     FIG. 3 is a section taken through a diametric plane of symmetry through the stress limiter of FIGS. 1 and 2; and 
     FIG. 4 is an elevation, in part section, of one half of the two-part stress limiter of FIGS. 1 to 3. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An optical aerial cable 10 has entirely non-metallic components including an outermost cylindrical plastic sheath, and is suspended as a continuous length between towers. At each tower, a short length of the cable 10 is supported by a support fitting 12 connected mechanically and electrically to the tower (not shown). The support fitting 12 comprises a layer of steel wires wound helically around the cable in a tight grip. A stress limiter 11 is held between the cable and the ends of the wires. 
     The stress limiter 11 is a mushroom-shaped hollow, stainless steel preform in two identical halves, one of which is shown in elevation in FIG. 4, conveniently formed from sheet metal. The stress limiter has a tubular stem portion 13 from which depends a generally part-toroidal head portion 14 with a smoothly-curved outer profile for minimizing electrical stress. The stem portion 13 has a slightly greater wall thickness over most of its length, this region of the stem portion being longitudinally splined on its outer surface as shown at 18 in FIG. 4. The stem 13 has a smooth end section 131 which is of smaller diameter, for ease of assembly. 
     There is at 16 a narrow annular gap between part 15 of the head portion 14 and the splined tubular stem 13, to allow for the insertion of the ends of the wires of the support fitting 12 into the head portion. The dimension of the gap is predetermined to suit the diameter of the cable and the diameter of the wires. Once inserted, the wires are retained axially by the barb effect of the sharp edge of the head portion 15 against which the wires engage. This barb effect is enhanced by the angle of inclination of the re-entrant edge of part 15 which is conveniently about 45° to the axis of rotational symmetry of the stress limiter. 
     The stress limiter is fitted in situ after fitting the support fitting 12. The end region of the support fitting 12 is partially loosened by slightly unwinding the wires, and both halves of the stress limiter are inserted with the tubular stem portions 13 pushed fully into the gap between cable 10 and wire ends of the support fitting 12, the wire ends extending fully into the toroidal space inside the head portion 14 of the assembled stress limiter. The stress limiter is then rotated manually, the splines of the stem portion 13 gripping the wire ends, to re-tighten the support fitting around the cable 10. The assembly is then stable and the load on the support fitting ensures that it will not unwind. 
     The smooth, cylindrical inner surface of the stem portion 13 spreads the mechanical load on the cable, reducing mechanical stress. 
     Conveniently, the wall of the head portion is provided with a plurality of holes spaced at different positions around its peripheral region, for example, as at 17, so that at least one of the holes lies at or adjacent the bottom of the stress limiter, and acts as a drain hole when the stress limiter is fitted to a cable. In some cases however the stress limiter may have a single drain hole only, the stress limiter then being fitted to the cable with the drain hole lowermost.